Photothermographic film generally comprises a base material, such as a thin polymer or paper, typically coated on one side with an emulsion of heat sensitive materials. Once the film has been subjected to photostimulation, such as via a laser of a laser imager, for example, a thermal processor is typically employed to develop the resulting latent image through application of heat to the film. In general, a thermal processor raises the base material and emulsion to an optimal development temperature and holds the film at the development temperature for a required time period to develop the image. To provide optimal and consistent quality in developed images, a thermal processor must perform this heating operation smoothly and consistently within a single film and between multiple films.
One type of processor for thermally developing photothermographic is typically referred to as a drum processor. One type of drum processor employs an internally heated rotating drum having a series of non-heated pressure rollers positioned around a segment of the drum's surface. During development, rotation of the drum draws the photothermographic film between the drum and the pressure rollers, with the pressure rollers holding the film, typically the emulsion-side, in contact with the drum as the film moves through the processor. As it moves through the processor, heat is transferred to the film and it is heated to an optimal development temperature to develop the latent image.
While heat is transferred to the photothermographic film primarily from the heated drum, some heat is also transferred to the film from the non-heated pressure rollers. During idle times, when film is not being processed, the pressure rollers are in direct contact with and absorb heat from the heated drum. As film passes between the drum and pressure rollers during processing, a portion of this heat is transferred to the film. At low film throughput (i.e. the number of films processed in a given time period), heat transfer from the pressure rollers typically does not pose a problem as heat transferred to given sheet of film is recovered through contact with the drum between sheets so that the temperature of the rollers does not significantly drop.
However, at higher film throughput (such as continuous film feed, for example), the pressure rollers are not able to recover heat from the drum between films, and the temperature of the rollers, particularly those which make first contact with the film, decreases with successive films until an equilibrium or steady state temperature is reached. Consequently, earlier films of a series of films being processed have different temperature profiles and absorb more heat than later films of the series, resulting in uneven densities of the developed images between films of the series.
In view of the above, there is a continuing need for improved photothermographic film developers. In particular, there is a need for a thermal processor that reduces variations in image density resulting from variations in roller temperatures as described above.